Align plugin unit tests guide with current CI options

[ekamran]

Summary

Updates the plugin unit testing guide so it matches the current wp scaffold plugin-tests documentation.

Changes

• Replace Travis CI-specific wording with general continuous integration wording.
• Document CircleCI as the default generated CI configuration.
• Add a GitHub Actions example using --ci=github.
• Update generated file references to match the current scaffold docs.

Verification

• Ran composer lint
• Ran composer phpcs
• Ran composer phpunit
• Ran composer behat (skipped: no behat.yml)
• Ran git diff --check

Note: composer phpstan currently fails on existing type errors in bin/Handbook_Command.php unrelated to this documentation change.